Chapter 9 Flashcards
what is a chemical bond
a strong attractive force that exists between neighboring atoms in a substance
three types of chemical bonds
- ionic
- covalent
- metallic
what is a metallic bond
a regular arrangement of metal atoms, but the valence electrons move throughout the crystal
what is an ionic bond
a chemical bond formed by the electrostatic attraction between positive and negative ions
when do ionic bonds form
when one or more electrons are transferred from the valence shell of one atom to the valence shell of another atom
the atom that transfers electrons in ionic bonds
cation
the atom that gains electrons in an ionic bond
anion
G.N Lewis
introduced the Lewis Electron-Dot symbol to describe ions and molecules by considering a “cubical atom”
G.N Lewis and his hypothetical “cubical atom”
there is space for 8 electrons at the corners of a hypothetical cube
what did G.N Lewis recognize about atoms
there is a “kernel” of electrons that remain uncharged, and an outer shell that varies during chemical reactions between 0 and 8
valence electrons
the outer shell electrons of an atom. they are the electrons that participate in chemical bonding
Lewis’s Cubical Atom (definition)
a cube provides a simple means to illustrate valence electrons around an atom, but this is more commonly represented in 2-dimensions using “dots” to represent the valence electrons
Lewis dot symbol
a notion in which the electrons in the valence shell of an atom or ion are represented by dots placed around the chemical symbol of the element
how are lewis dot symbols placed
dots are placed one to a side until all 4 sides are occupied
what concept did Lewis introduce
the electron-pair bond and the “group of 8” electrons being the favorable situation
what did Lewis’s concept lead to
-the octet rule
octet rule definition
-molecules or ions tend to be most stable when the outermost electron shells contain 8 electrons and have structures that are isoelectronic with the noble gasses
what does the combination of ionization energy and electron affinity need
it needs energy because it is endothermic
lattice energy
the change in energy when an ionic solid is separated into gas-phase ions
what happens when 2 ions bond
energy is released, making the overall process exothermic
according to Coulombs law, what 2 factors affect the strength of the ionic bond
- charge (Q)
- size of the ions
Coulomb’s Law
the energy (E) obtained in bringing 2 ions with electric charges Q+ and Q- from infinite separation to a distance (r) apart
equation for coulomb’s law
E=(Q+ x Q-)/r
Q+
the charge on the cation
Q-
the charge on the anion
r
the distance between the ions
what does solving for E in coulomb’s law give you
the strength of the ionic bond
ionic substances are typically ________ solids
high-melting
what 2 factors affect the strength of the ionic bond (Melting point: Coulomb’s Law
melting point (strength of ionic bond) increases as Q increases and/or as r decreases
covalent bond
a chemical bond formed by sharing a pair of electrons
how do we understand how covalent bonds form
- we monitor the energy of two isolated hydrogen atoms as they move closer together
- the energy decreases, first gradually and then more steeply, to a minimum
- as the atoms continue to move closer, energy starts to increase dramatically because the nuclei repel
bond length
the distance between the atoms when energy is at a minimum
what is the importance of the Born-Haber Cycle
because it is very difficult to measure the lattice energy directly, using the energy changes for steps helps make it easier and gives the same result
Orbitals
a place to put electrons
Elements to the left of carbon have __________ and those to the
right have ________
empty orbitals
filled orbitals (lone pairs)
coordinate covalent bond
is one where a single atom provides
both electrons
Bond Lengths sorted by size
Triple bond < Double Bond < Single Bond
Ionization energy and electron affinity provide
an
indication of how readily an atom may want to give up or
accept an electron
Electronegativity is considered to be
the ability of an atom
in a molecule to attract electrons to itself.
Linus Pauling
quantified the concept of electronegativity by comparing the
bond energy of a heteronuclear A-B bond with the corresponding
homonuclear A-A and B-B bond energies.
The bond energy E(A-B) for a molecule A-B is always
greater than
the mean of the bond energies
Pauling argued that the excess bond energy is due to the
ionic
component caused by the partial charges on the atoms in AB.
Electron Affinity versus Electronegativity
Electron Affinity: measurable, Cl is highest.
Electronegativity is the ability of an atom to attract toward
itself the electrons in a chemical bond.
Electronegativity: relative, F is highest
when the difference in electronegativity is 0
the bond is covalent
when the difference in electroegativity is greater than or equal to 2
the bond is ionic
when the difference in electronegativity is between 0 and 2
the bond is polar covalent
Formal charge results from assuming
equal sharing of the
bonding electrons
The formal charge on an atom in a Lewis dot structure is
determined by inspecting
how many lone pairs are present and
how many bonds it forms.
The sum of the formal charges of the atoms in a molecule or ion
must equal
the charge on the molecule or ion.
Resonance
means that the actual structure is a composite or an
average of the two structures.
classic example of “resonance”.
benzene
A “circle” is used to depict the fact that
the bonding cannot be
properly represented by a single Lewis structure in which a
“line” represents a shared electron pair bond
Exceptions to the Octet Rule
- The incomplete octet
(hypovalent or electron deficient molecules). - Odd-electron molecules
- Hypervalent molecules (“valence shell expansion”)
Bond order
is the number of pairs of electrons in a bond.
bond energy
The enthalpy change required to break a particular bond into one mole of
gaseous molecules
Bond energy is a measure of
bond strength: the greater the bond energy,
the stronger the bond, the more stable the molecule
Bond energies can be used to estimate the
enthalpy change, ΔH, for a
reaction
